Le minerai de Fer fort des plateaux du Bajocien des régions frontalières du Pays-Haut (France), de la Gaume (Belgique) et du Gutland (Grand-Duché de Luxembourg)

The “Fer fort”, a non-oolitic ironstone located in the Eastern Paris Basin on the top of the Dogger cuesta in the border regions of France (Pays-Haut), Belgium (Gaume) and Grand-Duchy of Luxembourg (Gutland), was already used during the Iron Age, together with Jurassic oolitic ironstones. The “Fer fort” ironstone corresponds to the Borne de Fer ferricrete and clastic blocks of ferricrete reworked in karstic cavities of the Dogger plateau and in the entrenched valleys. Ferricretes are the results of a subtractive weathering profile on upflipted regions under climates with tropical affinities, during Early Cretaceous or/and Palaeogene periods. The “Fer fort” ironstone has numerous features specified by thin sections and chemical and mineralogical analyses, but it is almost characterized by the presence of goethite, the great content of Fe (generally more than 50 %), and the lack of phosphorus allowing its use before the Industrial era.